[TowerTalk] Inverted L.....thanks for the input folks

[Jim Lux]

Hmm..

Looking at the ground system losses, I'll agree with Mike (and Tom) that not having a nice radial field in suburban environments is a problem, but...

Here's a thought:

one function of that ground is to couple the bottom of the antenna to "earth" (i.e. provide a low impedance connection).. for that, symmetry is probably less important than sheer area and number of wires. Consider each wire as a plate of a capacitor to ground. The more the better.

Another function is to reduce the losses in currents induced in the surface of the soil as the near field, for that, you'd want, zeroth order, a nice symmetric sheet of low resistivity metal with radius comparable to antenna height.

Then, there's the whole "far field" thing with surface reflection of vertically polarized antennas (yep.. heading on out to the Salton Sea or the beach seems a good idea.. maybe that's why houses in Malibu on the beach cost so much.. better low band propagation.. who'd a thunk it?)



yes, they're sort of the same function.. But the "low resistance ground connection" could theoretically be done by a buried web of wires that's deep, or by burying the wires in concrete (which is fairly low resistance, compared to most soil.. yes.. that reinforcing mesh has more than one use!).  But the "low resistance for induced currents" sort of requires a physical presence in all locations..

Jim, W6RMK




-----Original Message-----
>From: Jim Brown <jim at audiosystemsgroup.com>
>Sent: Dec 26, 2007 4:34 PM
>To: "towertalk at contesting.com" <towertalk at contesting.com>
>Subject: Re: [TowerTalk] Inverted L.....thanks for the input folks
>
>Tom, W8JI posted this in response to a similar question on the 
>Topband list. Like anything Tom has to contribute, it's worth 
>repeating. One of the most difficult things to get your head around 
>is the RELATIVE importance of various factors. Tom does a very good 
>job of articulating that here. A few hours later, W4EF, another 
>VERY sharp engineer added his comment. I agree with both of these 
>guys.
>
>73,
>
>Jim Brown K9YC
>
>On Wed, 26 Dec 2007 09:51:27 -0500, Tom Rauch wrote:
>
>>The most difficult to deal with loss in a vertical antenna, 
>>once it is 1/8th wave tall or taller, is in the ground 
>>system. While it is possible to do something wrong, like 
>>make the antenna out of a thin resistive conductor or use an 
>>exceptionally bad loading system design, if even reasonable 
>>care is taken or with reasonable selection of materials most 
>>of the loss is in the ground system.
>
>>If the ground system is very good then there is almost no 
>>difference at all between top loading, center loading, or 
>>base loading. The exception would be a poor loading coil 
>>design, for example a coil that was approaching 
>>self-resonance at the operating frequency. A loading coil 
>>self-resonant on 4 MHz would not be a good loading coil on 
>>160 no matter how the inductor was constructed.
>
>>Follow the rules of common sense...use a big hat or 
>>something with some self-capacitance near the top. Use 
>>space-wound turns and a good form factor on the coil with 
>>reasonable size wire. Use a good ground system. The result 
>>will be almost no difference at all between a 150 foot 
>>vertical and a 50 foot vertical.
>
>>Do things wrong and height can make a great difference.
>
>>73 Tom
>
>
>On Wed, 26 Dec 2007 13:54:29 -0800, Michael Tope replied:
>
>>Tom, 
>
>>One thing to consider is that many folks who are using short
>>verticals are doing so because they live on small lots. Because
>>of this a good ground system isn't always practical. I've paved 
>>or will be paving every square inch of my small lot with radials, 
>>but even when I got the last little patch of ground covered better 
>>than I have now, I'll still have a compromised ground system
>>compared to someone who can run full length radials with 
>>complete azimuth coverage. Given those limitations (again very 
>>common to city dwellers), I think that there is some worthwhile 
>>advantage in chosing top-loading over base loading. 
>
>>I would agree that with a good ground system (30 to 60 1/4 wave
>>radials with fairly uniform angular spacing), the choice of 
>>loading technique won't make that much difference. 
>
>>73, Mike W4EF..................................
>> 
>
>
>_______________________________________________
>
>
>
>_______________________________________________
>TowerTalk mailing list
>TowerTalk at contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk